The present invention relates generally to adaptive transversal equalizers, and more specifically to an adaptive transversal equalizer capable of operating at the speed of digital communications systems.
Transversal equalizers are used for equalization of a digitally demodulated signal, and development efforts are being directed to digitizing transversal equalizers to meet the recent requirements that include compactness, low power consumption and free of adjustments.
Prior art all digital transversal equalizers comprise a digital transversal filter and a tap-weight control circuit, or correlator. The digital transversal filter includes a tapped-delay line to which a digital signal is supplied from the A/D converter of a demodulator. The tapped-delay line has (2M+1) successive taps (where M is an integer equal to or greater than unity) to which multipliers are respectively connected to multiply the symbols at the taps with respective tap-weight control signals supplied from the control circuit, the outputs of the multipliers being summed by an adder to produce an equalized signal. The correlator includes (2M+1) exclusive OR gates for detecting correlation between the data and error components contained in the equalized output, and integrators for integrating the outputs of the exclusive OR gates for a certain symbol interval and averaging the integrated values to produce the tap-weight control signals to adjust the tap weights at the intervals of the symbols supplied to the tapped-delay line to adaptively equalize the data signal.
However, there is an upper limit that is imposed on the maximum operating speed of the correlator and on the maximum operating speed of LSI circuits that comprise the multipliers and adder of the transversal filter. Since the recent digital communications system demands ever-increasing speed which is higher than the operating speed of the current LSI circuit, the prior art all digitized transversal equalizer is not capable of meeting the demand of the digital communications system. Therefore, with the current digital technique, the integrators of such correlators have to be implemented in analog form to meet the high speed requirements of the digital communications system.